This invention concerns a device designed for continuous measurement of variations in the linear mass of textile products such as thread, wicks, ribbon etc.
This type of device is already known in principle for the detection of variations in cross-section, or the presence of areas of greater thickness in threads, in view of eliminating them. It basically consists of passing the product between the two plates of an air capacitor, using as a basis for measurement the variations in its capacitance in terms of the cross-section of the product. These variations are compared to the fixed capacitance of a reference capacitor. They are then amplified and processed by a micro-computer as shown in FIGS. 4 and 5 of the applicant's French Pat. No. 83/10360.
One disadvantage of this device is the need, due to the fact that the variations are very small, for very highly mechanically and electrically stable measurement electrodes forming part of the capacitor plates. In the case of threads of very differing grades, the use of pairs of electrodes has been proposed that are separated by equally different spaces. The device then comprises, as shown in FIGS. 2 and 3 of the French Pat. No. 71/24828, a number of fixed pairs of electrodes selected according to the different products to be tested or according to the different grades of a single product.
The variations in capacitance between each pair of electrodes, which are due to the variations in the dielectric constant of the thread passing between them, are transformed into electrical signals by connecting the capacitor to a resonant circuit, whose variations in frequency of resonance can be monitored.
The inherent difficulty in this system is the problem of introducing and extracting each product into and from the appropriate test corridor and this represents a certain limitation in use as well as a possible cause of error. In addition, the juxtaposition of the electrodes can cause interactions with a detrimental effect on the quality of the measurement.